1. Field of the Invention
The invention is related to in-service sensors for detecting, locating and assessing strain, crack, deformation and internal void characteristics of a composite or metallic structure.
2. Description of Related Art
Typically, structural support members placed in service are subjected to structural static and dynamic loads which result in measurable strain. Static, fatigue and impact service loads may eventually produce permanent deformation and/or cracks in metallic structures, and cracks and/or internal voids in composite structures over the life of the support. Deformation, stressing and degradation of the molecular structure occur during the life of the support members throughout the volume of its structure. An accurate ascertainment of the state of any physical support at any given time usually requires testing of the structural member and/or a microscopic view of a cross section of the structural member. Costly ground based non-destructive test methods such as X-Ray, eddy current and ultrasound inspections do not provide adequate information on the integrity of all sections of interest or a cost effective method to do so on a periodic basis.
However, it has been shown that defects occurring within a structural member are related to surface defects and acoustic emission at the surface, and therefore an analysis of defects and acoustic emission at the surface should provide a satisfactory indication of the integrity of the structural member. A visual examination of the surface of a structural member typically requires the in situ location of microscope devices and the ability to physically reach and prepare the surface being observed.
The surfaces which are structurally known to experience the greatest amount of stress under loading conditions will contain cracks, voids and permanent deformation which may not be in an accessible location. In addition, the time and effort and regularity required for human surface assessment is highly prohibitive of surface measurement, which is dangerous when the structure is not replaced after the onset of defects, and yet wasteful when the structural member is replaced based upon an average or mean time between failure analysis.
In response to these physical and economic considerations, some strain and crack sensors have been formulated which perform local area stress evaluation over a large area. Existing acoustic emission sensor technology has been developed which inherently provides large area coverage using passive and active monitoring signal processing techniques. Crack, strain and acoustic emission sensors would be integrated with supporting electronics on a conformal circuit which transmit processed structural characteristics to a central processing location for measurement and analysis. Current strain and crack gauge technology is limited to small area gauge clusters with no provisions for connectorization to external systems or interconnection to collocated signal processing hardware. Current acoustic emission sensor technology is limited to discrete sensor packages with collocated signal amplification with no provisions for multiple sensor configurations or collocated signal processing hardware. This general scheme is somewhat burdensome and necessitates the extension of a large number of interconnects transmitting unprocessed data at high data rates to a central location. What is needed is a conformal circuit capable of mounting with crack, strain and acoustic emissions sensors which has the bulk of the optical and/or electrical interconnects at the site of the conformal circuit for ease of installation. Electrical interconnects at the conformal circuit will accommodate existing strain, crack and acoustic emission sensor technology, whereas embedded photonic interconnects are envisioned for emerging optical sensor technology, signal processing and data bus transmission. Such a circuit would perform sensing analysis locally and only transmit a summary of the significant conditions at the sensors through a set of optical and/or electrical data lines to a central processor for further evaluation.